A system for monitoring the pressure of a tyre of a vehicle wheel comprises at least - inserted in the wheel rim - a pressure sensor, a transmitter and a battery and a power-supply capacitor for the transmitter.
The system must regularly transmit, to the central computer of the vehicle, bursts of bits of information relating, of course, to the pressure of the tyre but also to, for example, the temperature, the voltage of the power-supply battery and the identity of the wheel.
The system must transmit a specific number Ne, generally five, of identical bursts and a number Nr of these bursts must have been received so that it may be considered that the immediate information has been transmitted and received. The transmission of the bursts can take place, for the sake of providing a basis, every hour if the vehicle is stationary, and every six minutes if the vehicle is moving.
Furthermore, the system is activated at regular intervals, for example, every two seconds, for the transmission of the bursts. Finally, the bursts, each of a duration t.sub.b, generally of 8 ms, are spaced, again generally by 12 ms. Thus series of five bursts are transmitted, of a duration of 88 ms over an activation period of 2s.
If it is desired that the system be resistant to very low temperatures, of the order of -40.degree. C., it is necessary to use expensive power-supply batteries, costing around 2 Euros.
The Applicant has sought to overcome this constraint by using inexpensive batteries costing less than 1 Euro but which, consequently, are not made for such low temperatures, such as, for example, recyclable lithium batteries.